Setting output power levels of transmitters, base stations in downlink and mobile stations in uplink in mobile communication systems is commonly referred to as power control (PC). Objectives of power control include improved capacity, coverage, user quality (bit rate or voice quality), and reduced power consumption.
Power control can be divided into different aspects:                intra-cell power control, taking into account information from and the relation to the serving cell only; and        inter-cell power control, taking into account the relation to and information from neighbour cells, in addition to intra-cell power control.        
FIG. 1 illustrates a problem that arises in cellular mobile communications systems, whereby mobile terminals associated with one cell interfere with communications of another cell.
Thus, FIG. 1 shows a first mobile terminal (also known as a user equipment) 2 which belongs to a first cell (not illustrated) maintained by a radio base station 4. Data and control signals are transmitted from the radio base station 4 to the mobile terminal 2 in the downlink, and in the reverse direction in the uplink. Signals transmitted from the mobile terminal 2 in the uplink have a signal power of S1. A similar arrangement exists in a nearby, possibly neighbouring, cell, where a second mobile terminal 6 communicates with another radio base station 8. Signals transmitted from this mobile terminal 6 in the uplink have a signal power of S2.
Particularly when the first mobile terminal 2 is near the edge of its cell, convention dictates that its transmitting power is set high, to ensure that the transmitted signal can be received correctly by the associated radio base station 4. However, in these circumstances, transmissions from the first mobile terminal 2 may interfere with communications in the nearby cell. Thus, a signal I causes interference in communications between the second mobile terminal 6 and its associated base station 8.
A communication path 10 exists between the two radio base stations 4, 8. In some telecommunications systems, this is a direct interface (for example, in evolved UTRAN, known as the X2 interface); in other systems, the communication path may be indirect, requiring communication via a core network.
There are several suggestions for providing inter-cell uplink power control, which will put constraints on the power usage and reduce the serving cell desired power (the intra-cell power control). The main suggestions relate to use of an overload indicator (OI), where a nearby cell that is receiving interference can inform the cell that is causing the interference of this fact.
For example, in evolved UTRAN (E-UTRAN), the radio base station experiencing interference sends an OI over the X2 interface to the relevant radio base station. This radio base station can then control the interfering mobile terminal accordingly. In WCDMA, the radio base station experiencing interference sends a relative grant directly to the mobile terminal that is causing the interference (even though it is sending data to a different cell).
An obvious response to receipt of an OI is to reduce or limit the transmission power of the mobile terminal. In this way, interference with nearby cells may be reduced. However, the system response to receipt of an OI has not yet been standardized in LTE. In WCDMA, the mobile is specified to reduce the transport format and power according to the relative grant from the neighbour cell.